Tunnel barriers based on rare earth element oxides

Abstract

Magnetic tunnel junctions are disclosed that include ferromagnetic (or ferrimagnetic) materials and a bilayer tunnel barrier structure that includes a layer of a rare earth oxide. The bilayer also includes a layer of crystalline material, such as MgO or Mg—ZnO. If MgO is used, then it is preferably (100) oriented. The magnetic tunnel junctions so formed enjoy high tunneling magnetoresistance, e.g., much greater than 100% at room temperature.

Description

TECHNICAL FIELD [0001] The invention relates to an improved tunnel barrier for use in spintronic devices such as injectors of spin polarized current and the magnetic tunnel junction (MTJ). MTJ magnetoresistive (MR) devices find use as magnetic field sensors such as in read heads for reading magnetically recorded data, as memory cells in nonvolatile magnetic random access memory (MRAM) cells, and for magnetic logic and spintronic applications. More particularly, this invention relates to a method of forming improved composite tunnel barriers formed from rare-earth oxides and oxides of Mg, Al and Zn. BACKGROUND OF THE INVENTION [0002] The basic component of a tunnel spin injector and a magnetic tunnel junction is a ferromagnetic layer combined with a tunnel barrier. The basic structure of an MTJ is a sandwich of two thin ferromagnetic and / or ferrimagnetic layers separated by a very thin insulating layer. In both the spin injector and the MTJ, the electrons that tunnel from the ferroma...

AI Technical Summary

Benefits of technology

[0014] The MgO and Mg—ZnO tunnel barriers of the magnetic tunnel junction devices disclosed herein are preferably prepared according to methods in which the lower ferromagnetic (or ferrimagnetic) electrode is not oxidized, so as to give much higher tunnel magnetoresistance values than in the prior art using other tunnel barrier material such as aluminum oxide. Similarly, much higher spin polarization values of tunneling current are obtained in tunnel junction devices with one or more ferromagnetic (or ferrimagnetic) electrodes. The MgO or Mg—ZnO tunnel barrier so formed does not have a significant number of defects that would otherwise lead to hopping conductivity through the tunnel barrier. In preferred methods, highly oriented (100) MgO or Mg—ZnO barriers are formed without using single crystalline substrates or high deposition temperatures, thereby facilitating the manufacture of devices using standard deposition techniques on polycrystalline or amorphous films. Post anneal treatments are preferred to improve the tunneling magnetoresistance, which for the MgO structures disclosed herein can exceed 50, 100, 150 or even 200% at room temperature, and which for the Mg—ZnO structures disclosed herein can exceed 50% at room temperature.

[0015] For several aspects and embodiments of the invention disclosed herein, a MgO or Mg—ZnO tunnel barrier is sandwiched between an underlayer and an overlayer, either one or both of which may include one or more layers of a ferromagnetic material and / or a ferrimagnetic material. While the MgO (or Mg—ZnO) tunnel barrier is preferably in direct contact with the ferromagnetic material and / or ferrimagnetic material, each of the underlayer and overlayer may optionally include one or more spacer layers which are adjacent to the tunnel barrier but which do not significantly affect the tunneling properties of the MgO (or Mg—ZnO) layer, e.g., by not significantly diminishing the spin polarization of electrons tunneling through the tunnel barrier. For example, Au or Cu may be used as non-magnetic spacer layers or the spacer layer may be comprised of a conducting oxide layer. (It should be understood that the terms underlayer and overlayer do not necessarily imply any particular orientation with respect to gravity.) Performance of the MgO (or Mg—ZnO) tunnel barriers disclosed herein may be improved through annealing, wherein performance refers to various attributes of the tunnel barrier or associated device. For example, annealing a magnetic tunnel junction improves, in particular, its magneto-tunneling resistance; annealing a tunnel barrier improves, in particular, its spin polarization. In particular by annealing these tunnel barriers, tunneling magneto-resistance of more than 100% can readily be achieved using methods of thin film deposition and substrate materials compatible with conventional manufacturing technologies. Annealing temperatures may be in the range from 200° C. to 400° C. or even higher; however, the best tunnel barrier performance was obtained for annealing temperatures in the range from 300° C. to 400° C. The same anneal that improves the tunneling magnetoresistance may also be used to set the direction of an optional exchange bias field provided by an antiferromagnetic exchange bias layer and may also be used to set a direction of a uniaxial magnetic anisotropy in the magnetic electrodes.

Problems solved by technology

A potential disadvantage of crystalline MgO tunnel barriers is that the magnetic properties of the free or sensing magnetic layer, adjacent to the MgO barrier, may be influenced by the crystallinity of the MgO layer, leading possibly to greater variations in magnetic switching fields, from device to device, than are seen using amorphous barriers with no well defined crystallographic structure.

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